Divide et impera
The Roman maxim Divide and Conquer also dictates any successful solution of the Navier-Stokes equation – the root of computational fluid dynamics and one of the few design methodologies common to all four America’s Cup teams racing in New Zealand. So a good moment to revisit the basics with Manuel Fluck
The elevator pitch Computational Fluid Dynamics (CFD) is a very powerful asset in the engineering tool- box for modern sailboat design. Actually, today CFD is so powerful that it has often replaced physical wind tunnel or tank testing. But how can something as compli- cated as the wind flow around two sails be even computed? How does CFD work and what are its limitations? I have tried here to deliver the basic introduction but with- out the maths!
A little more depth When dealing with boat performance analysis or boat rating, one will probably have come across CFD. Some may well have read further about RANS simulations, VLM or other such incarnations of CFD
50 SEAHORSE
which we will look at soon. For some CFD might just be an obscure term the engineers throw around. Either way, Computational Fluid Dynamics is at the core of much work conducted for our sport. For example, when analysing sail forces and optimising sail shape, or assessing hull drag and the efficiency of a foil, CFD now lets us (engi- neers) calculate the forces air and/or water exert on the boat with great accuracy. While CFD is usually much quicker
than any physical (wind tunnel or tank) experiment, it also lets us analyse the com- plicated flow around all parts of the boat such as sails, rig, hull… at a level of detail that is hardly achievable in any physical experiment. One reason is that it is rela- tively easy to zoom into a computer simu- lation to investigate details, whereas a close-up look at a physical experiment is often limited. Think only of the measure- ment instruments used and their interac- tion with the flow – even in the most sophisticated wind tunnel. Thus CFD gives us a superb opportunity to understand in great detail what is going on and to improve our designs relatively quickly. On the other hand, physical experi-
ments are intuitive: if you put two sails into a wind tunnel it is pretty obvious what is going on. You trim the sails, you measure forces and maybe the pressure. Executing a wind tunnel experiment accu- rately is surely not easy, but the process is transparent and seems easy to follow.
CFD, on the other hand, is a bit of a
magical black box. One throws in some model and magically gets out a lot of data. But what is happening in between? How does CFD generate data that can replace wind tunnel experiments? CFD is indeed a powerful tool. How-
ever, with power comes responsibility: the responsibility of the CFD engineer to choose the appropriate tools, and the responsibility of the CFD consumer (project manager, boat owner, or simply interested sailor) to have at least some basic understanding of the amount of truth contained in the pretty pictures generated by the CFD tool. Now let us scratch the surface of this black CFD box and try to get some insight into what is going on behind the scenes…
Some history Already in the early 1800s a French engi- neer and physicist, Claude-Louis Navier, and an Anglo-Irish physicist and mathe- matician, George Gabriel Stokes, indepen- dently developed the famous Navier- Stokes equation. This equation describes the motion of any fluid – which for our purposes is anything that is not solid. The solution of the Navier-Stokes equa-
tion provides everything we are interested in when talking about fluid dynamics: flow velocity, density and pressure at any point in space and any instance in time. However, I promised no maths. And I will
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